The classic omni-directional antenna is a vertically oriented half wavelength dipole as shown in FIG. 1. It produces a radiation pattern which has no variation in azimuth and an elevation pattern which is 78 degrees wide. Half wavelength dipoles typically have a bandwidth of 10 percent or less. In practice most omni-directional antennas have some azimuthal variation as shown in FIG. 2. This variation can be caused by the proximity of the a structure the antenna is mounted on, or asymmetries in the antenna.
According to standard 2.257 in the IEEE standards dictionary an omnidirectional antenna is defined as “An antenna having an essentially non-directional pattern in a given plane.” The IEEE standard does not specify how much variation an antenna pattern can exhibit and still be called omni-directional.
Conical dipoles as shown in FIG. 3, wherein the arms of the dipole are cones have perfect omni-directional patterns and can achieve bandwidths of a couple octaves. Printed circuit versions of dipoles and bicones incur some ripple in their azimuth patterns because they do not have rotational symmetry. The printed circuit version of the bicone antenna is called a bowtie antenna and is shown in FIG. 4.
The omni-directional antennas discussed above are vertically polarized. Horizontally polarized omni-directional antennas are less frequently employed. A common horizontally polarized antenna is a vertically oriented slot. Slots require a larger metallic structure to exist within such as a ground plane or a waveguide. Slots also require an external excitation such as a waveguide or a circuit that couples to the slot. Slots are rather narrow band—on the order of a few percent—and slots tend to have high impedances relative to the 50 ohm circuitry that is common used in microwave circuitry.
A simple slot antenna is the Alford slot antenna. It consists of a single thin longitudinal slot in a narrow cylinder as shown in FIG. 5. The Alford slot antenna has an omni directional horizontally polarized pattern when the diameter of the cylinder is less than ⅛ of a wavelength. When the diameter of the cylinder is greater than ¼ wavelength, the Alford slot has a directional pattern. The Alford slot shown in FIG. 5 has a diameter of ⅛ wavelength and a slot length of about ¾ wavelength. The ⅛ wavelength tube is so small that waveguide modes are completely cut-off inside the tube. However the tube loads the mode which exists in the slot, which accounts for the resonant length which is considerably longer than a half wavelength. The Alford slot shown in FIG. 5 is fed by a balanced high impedance transmission line. In practical usage, a balun would be required to convert the input to an unbalanced 50 ohm transmission line.
A numerical analysis by this writer showed that the Alford has about 2 dB variation in its azimuth pattern as shown in FIG. 6A. The bandwidth can be calculated from the notations on the Smith chart: 2*(3.0 Ghz−2.94 Ghz)/(3.0 Ghz+2.94 Ghz)=2 percent. If the impedance contour in FIG. 6B can be made to enclose the origin, the bandwidth can be increased. However it was difficult to make numerical simulations of the Alford slot converge, indicating that it is sensitive to small dimensional variations. The impedance contour in FIG. 6B is transformed from the port at the bottom of the antenna to the slot. The impedance contour resides mostly in the upper half of the Smith Chart indicating that the slot is inductive. The inductive nature of slots will be noted later in describing the operation of the antenna according to this invention. It will employ added capacitance to cancel the inductance of slots.
The slotted waveguide antenna shown in FIG. 7 has identical slots cut in the broadwalls which are adjacent to each other, parallel to the center line of the waveguide, and offset from the respective centers of the faces they reside in. Each slot radiates a somewhat directional pattern in opposite directions.
As noted above, slots are fairly high impedance radiators. Waveguide impedance increases with the width of the narrow walls. This leads to a trade-off with the slotted waveguide antenna. When the narrow walls are narrow, the patterns are better, but the impedance match is poor. When the narrow walls are wide, the impedance match is better but more ripple develops in the patterns. The radiation pattern of the slotted waveguide antenna shown in FIG. 7 are shown in FIG. 8. Note that the slotted waveguide antenna patterns have 1.5 dB ripple which is less than the Alford loop antenna. The waveguide shown in FIG. 7 is approximately one quarter height. If it were half height waveguide, the patterns would have about 4 dB ripple. The antenna according to this invention has less ripple.
Some improvements in the slotted waveguide omni antenna can be obtained by placing a coaxial feed network inside the waveguide as shown in FIG. 9. This alleviates difficulty of obtaining a good match with a thin guide. The impedance of the coaxial feed can be increased to improve the match even in thin waveguide. This may decrease the power handling capacity of the antenna. According to Ohmine, the slots should be centered to obtain omni directional radiation patterns.
The results of a simulation of Ohmine's antenna is shown in FIGS. 10A-B. FIG. 10A shows the impedance plot (showing only the band of frequencies for which the reflection is less than −10 dB) for Ohmine's coaxially fed slotted waveguide antenna, and FIG. 10B shows the corresponding simulated Azimuthal radiation patterns. This indicates the slotted waveguide antenna with coaxial feed has about 0.9 dB pattern ripple which is less than the waveguide fed slotted waveguide antenna. The patterns are better than those given by Ohmine, who indicates about 2 Db ripple in the horizontal polarization, and vertical polarization which is about 25 dB below the horizontal. This could occur if Ohmine used larger waveguide (scaled to the wavelength of the radiation). The bandwidth of the coaxial fed slotted waveguide can be calculated from the notations on the Smith chart in FIG. 10(b): 2*(3.03 GHz.−2.88 GHz.)/(3.03 GHz.+2.88 GAZ.)=5 percent. This is inferior to the waveguide fed slotted waveguide antenna. The rectangular tube can be reduced in size below the cut-off limit imposed by a waveguide feed as shown in FIG. 11. However, as the tube gets smaller, the resonant length of the slot gets longer Apparently the Ohmine antenna with a smallish tube behaves somewhat like the Alford slot antenna.